In controlling an internal combustion, spark ignited engine, for example in an automotive or other transportation vehicle application, the Engine Control Unit (ECU) monitors various parameters of the ignition and combustion events to optimize performance and minimize emissions created thereby. It has long been known that during the combustion event, the ionization produced in the cylinder or combustion chamber is capable of conducting electricity. As such, ion current sensors have been employed to provide the ECU with needed information regarding the initiation of the ignition as well as the quality and duration of the combustion event.
While the information provided by an ion current sensor is invaluable to an ECU in controlling the engine's performance, the requirement for the addition of a separate sensor and associated circuitry is cost prohibitive, particularly in the cost sensitive automotive market. In an attempt to reduce the overall cost and the attendant reduction in reliability that the addition of an extra sensor brings with it, engine control engineers have developed circuitry that allows sensing of the ion current utilizing electrodes of the spark plug that is already included to initiate the ignition event.
The ignition and ion sensing technology realized by these embodiments may also be used in diesel exhaust particulate filter regeneration. These systems utilize an active burner system that regenerates or burns the carbon build up in a diesel particulate filter (DPF) located in the exhaust of a diesel engine. One such system senses the pressure drop across the DPF. As the DPF collects diesel particulate carbon particles, the pressure drop across the filter increases. When the DPF pressure drop exceeds a predetermined threshold, the active burner system injects air and fuel into the engine's exhaust path up stream of the DPF and a spark plug driven by an ignition coil ignites the mixture. This creates a constant and controlled flame that is used to burn the carbon build up in the DPF (regeneration of filter).
The ion sensing feature is useful in detecting the presence of a flame in such DPF regeneration systems. It is dangerous to flow unburned fuel into the exhaust system if it is not being burned as intended because excessive levels of unburned fuel can collect and create an explosion hazard. Ion sensing provides a nearly instantaneous feedback on the status of the flame in the active burner system to ensure that the fuel being injected is burning as intended. Competitive technology is a thermocouple to sense the presence of flame, but the response time of the thermocouple is significantly slower than the ion sensing.
While such ion sensing systems provide extremely useful information for both reciprocating engines, so that the ECU may detect such conditions as engine misfire, combustion duration, engine knocking, approximate air/fuel ratio, indication of spark plug fowling, pre-ignition, etc., and continuous flame burners, so that the regeneration system can immediately detect the presence of flame, such systems typically require a second energy source to apply the voltage necessary to generate the ionization current. These systems also require a sensitive detection circuit for measuring the micro-amp ionization current, a signal which is particularly susceptible to electromagnetic interference (EMI) and other electrical noise related issues common in an engine environment. While the information obtained is quite valuable, the costs, complexity, and reduction and reliability resulting from the addition of such circuitry are extremely disadvantageous in most applications, and in particular in the cost sensitive automotive market.
It is desirable, therefore, for an ionization current detection circuit that does not require a separate high power voltage source or a micro-amp detection circuit. Embodiments of the present invention provide such circuitry.